Heterogeneous catalysts empower more than 90% of all chemical plants worldwide and play a key role in reducing environmental pollution. Common to all catalytic processes is catalyst deactivation and the associated loss of catalytic activity, which results in a decrease in production efficiency and increased maintenance costs. Catalyst deactivation is typically addressed by altering the composition and particle size of the catalysts. Although this will lengthen catalyst lifetimes, most catalysts still require periodic time off-stream for regeneration. One of the main deactivation mechanisms is the change of the catalyst oxidation state by oxidation or reduction (redox). However, if the catalyst could be tuned to favor the reduction under reaction conditions, the catalyst could actively reduce the oxide as it grew, alleviating the need for regeneration.
Recent studies have demonstrated that tuning of the crystallographic facets exposed at the catalyst surface significantly alters the nature of surface redox reactions. For example, during electro-oxidation of formic acid and ethanol, Pt nanocrystals with high-index {730} surface facets, which contain low coordination number atoms, are more active than low-index, {111} faceted Pt nanospheres. Similarly, water oxidation is more readily catalyzed by the (040) surface of BiVO4 than by (110) surfaces. Unfortunately, during such oxidative processes non-noble metal catalysts are often oxidized and deactivated. Although redox reactions can be tuned by catalyst faceting, the potential to drive in-operando healing by choosing appropriate catalyst surface orientations has not been explored.
A self-healing catalyst in oxidative environments would require the redox reaction on the catalyst surface be tuned to favor reduction of the oxide to the metallic state. Moreover, to take advantage of this functionality in heterogeneous catalysis, the redox reaction should be a side reaction rather than the primary one. For instance, in Fischer-Tropsch synthesis (FTS), the primary reaction involves the hydrogenation of CO and polymerization of hydrocarbons, with water produced as a side-product. Metallic Co is a superior FTS catalyst, but the presence of water, which reaches up to 40 vol % in industrial reactors, drives oxidative poisoning of the metallic Co. The oxidized Co is usually removed from the reaction stream and regenerated by hydrogen reduction. Thus, tuning the redox reaction to favor reduction of the oxide in-operando, would preserve the metallic Co under reaction conditions and constitute a key advance in the field of FTS.